Method for cross-trafficking between two slaves of an annular data network

ABSTRACT

In order to be able to maintain cross-traffic between slaves in a ring topology of an Ethernet-based data network even in the event of an error in the ring topology, which leads to the interruption of the ring topology, it is provided that in the event of the occurrence (rectification) of an error (F) in the annular data network ( 1 ), the ringmaster (RM) is configured to forward data packets or to block at least multicast data packets, and that the master (M) prompts the slaves (S 1 , S 2 ), communicating with one another via cross-traffic, to transmit configuration data packets (DPMC 1 , DPMC 2 ) as multicast data packets to each of the other slaves (S 1 , . . . , Sn) of the annular data network in order to adjust address tables (AT 1 , AT 2 ) in the slaves (S 1 , S 2 ), communicating with one another via cross-traffic.

This application claims priority under 35 U.S.C. §119(a) of AustrianApplication No. A50831/2015 filed Oct. 1, 2015, the disclosure of whichis expressly incorporated by reference herein in its entirety

The present invention relates to a method for data communication in theform of cross-traffic between at least two slaves of an annular datanetwork, wherein a master is connected to a first branch of the annulardata network with a number of slaves and to a second branch of theannular data network with a number of slaves, and the ends of thebranches are connected to a slave provided as a ringmaster.

In a data network, a network protocol is implemented, with which data istransferred in data packets in the data network between the networknodes which are connected to the data network. Probably the best knownand most widespread network protocol is the Ethernet protocol. Hereto,Ethernet defines data packets (also called data frame or Ethernetframe), in which data of a higher-level communication protocol can betransferred encapsulated in an Ethernet data packet. In doing so, dataof the communication protocol can be transferred in an Ethernet datapacket with a data length between 46 and 1500 bytes. Addressing in theEthernet protocol is effected by means of MAC (Media Access Control)addresses of the network nodes which are clearly allocated for everynetwork device. As seen from the perspective of the known OSI model,Ethernet is exclusively implemented on layers 1 and 2. In the higherlayers, different communication protocols can be implemented. Hereby, amultiplicity of communication protocols has been established, forexample IP in layer 3 or TCP and UDP in layer 4 to name but a few of themost widespread communication protocols.

With regard to hardware, today's Ethernet systems are so-called switcheddata networks, in which individual network nodes do not have to beconnected with one another and do not have to be able to communicatewith one another, but can instead be connected by means of couplingelements, for example network switches or network hubs. For suchpurpose, a coupling element has a number of network ports for the optionof connecting a network participant (either a network node or adifferent coupling element). Such a coupling element forwards anEthernet data packet either to all ports (hub) or to (one) specificport(s) (switch). Thus, so-called point-to-point connections are createdin a switched data network, in which Ethernet data packets are forwardedfrom one network node to a different network node by means of a numberof coupling elements.

In case of a network switch, a so-called Source Address Table (SAT) isimplemented in the switch. If the network switch receives a data packet,it stores the address of the sender (MAC address in Ethernet) and theport that received the data packet. As a result, the network switch canautomatically establish an allocation between addresses of network nodesand ports. This allows the network switch to specifically transmit adata packet via the port, through which, according to the source addresstable, a network node (or its address) can be reached in the network.This does not have to be configured because the source address table isautomatically established and maintained by the network switch. Entriesfrom this table are also automatically aged out again after a certaininterval if no further frames are observed from an already known networknode. Network switches with automatic source address tables are alsocalled unmanaged network switches.

Network nodes which are used in the industrial automation often have abuilt-in internal S-port switch, wherein two ports are accessible fromoutside and the third port serves the internal interconnection. As aresult, line topologies can be realized, in which a network node isconnected to the next adjacent network node in the form of a line, whichis advantageous in an industrial environment for reducing the cablingeffort. However, it is self-evident that external network switches orexternal network hubs can also be used for the setup of the networktopology. Basically, any network topology is possible, i.e. particularlya star topology, a line topology, a tree topology, a ring topology, etc.as well as any combination thereof. As a rule, a ring topology requiresspecific precautions in order to prevent the uncontrolled circulation ofmultiple-address data packets (multicast). For example, it is commonthat one network node is determined to be ringmaster, by means of whichno multicast traffic is transmitted. In the IT environment, protocols ofthe so-called spanning tree family have become prevalent, in which theswitches automatically recognize multiple paths to MAC addresses and insuch case do not use any paths except for one. However, the possibleswitching times for these protocols in the event of a ring break are notsufficiently short for industrial applications, particularly forreal-time applications.

In order to be able to also use Ethernet for industrial automation,real-time capable Ethernet protocols have already been developed becausethe standard Ethernet network protocol is known to not be real-timecapable. Examples of known real-time capable Ethernet protocols areModbus/TCP, Ethernet/IP, ProfiNET IRT, EtherCAT, or Ethernet POWERLINK,to name but a few. In this context, often also the terms industrialEthernet or real-time Ethernet are used. These real-time capableEthernet protocols are supposed to ensure data communication that issufficiently fast and deterministic for the corresponding application.They are thus supposed to ensure that a real-time relevant data packetis transferred via the network within a predetermined interval fromsender to receiver. In an industrial automation environment, real-timecapability means, e.g. that a fixed interval must be observed betweenthe acquisition of a measured value, transfer to a control unit,calculation of an actuating value in the control unit based on themeasured value, and transfer of the actuating value to an actuator forexecuting an operation. With reference to the real-time capable Ethernetdata network for transferring these data, a predetermined interval mustalso be ensured.

In an industrial automation environment, there is generally as least onemaster network node (hereinafter also called master for short) whichcommunicates with at least one associated, but usually a plurality ofassociated slave network nodes (hereinafter also called slaves forshort). For realizing a real-time capable Ethernet data network, theknown real-time capable Ethernet network protocols have defined apredeterminable cycle time, within which the master can usuallycommunicate with each slave. This comprises cyclically the possibilityof a data packet from the master to every slave and conversely also adata packet from each slave to the associated master. The attainable andbeforehand ascertainable minimal cycle time results from the sum of therun times of the data packets. Aside from the influence of theimplemented communication protocol, the run times are hardware-dependentand result from bit transmission times (length, payload) of the datapackets, network infrastructure (delays due to coupling elements), andthe network topology. The above-mentioned limits regarding the size ofthe Ethernet data packets must also be taken into account. This cyclicaldata traffic, which is the basis of the real-time capability in thereal-time capable Ethernet network protocol, is, as a rule, extended byasynchronous (non-cyclical) data packets in every transmission cycle.Such asynchronous data packets are used by the data communication, whichis not subject to the real-time requirements, for example, for theconfiguration of the slaves or for status queries. For such asynchronousdata packets, bandwidth is reserved, i.e., a specific, defined time forasynchronous data traffic is available in every transmission cycle.However, the network nodes must share this asynchronous section of atransmission cycle. The known real-time capable Ethernet protocolsdiffer with regard to the concrete implementation of the cyclical andasynchronous data traffic.

In the industrial environment, for example, in automation engineering,often also a redundancy is required, ensuring that the underlying datanetwork does not malfunction in case of an error, such as a cable breakor a defective network node. Therefore, due to the possibility of anetwork redundancy, ring topologies are of particular interest in theindustrial environment because every network node can basically bereached by two different paths. If the network is physically interruptedat some location, e.g. due to a cable break, disengaging of a plugconnection, etc., it does not necessarily result in the failure of theentire data network or even the subnetwork “behind” the breakage. Inorder to be able to maintain data traffic in the network with ringtopology even in the event of an error, methods which handle such errorshave already become known.

There are known methods for handling errors in the network topology,e.g. the known spanning tree method or the media redundancy protocolmethod. However, these methods, which were not designed for industrialapplication and much less so for real-time capable networks, are usuallymuch too slow and thus cannot be used in such applications. For theindustrial application, methods have been developed which allow for aquick reconfiguring of the data network.

The best known international standards, which describe implementationinstructions for ring topologies on the basis of Ethernet, are describedin IEC 62439, such as PRP (Parallel Redundancy Protocol) or HSR(High-availability Seamless Redundancy). In both cases, there are tworedundant paths between sender and receiver of a data packet, whereinthe sender must be capable of transmitting two specifically marked datapackets. The receiver must be capable of receiving these specificallymarked data packets and select one of them for further processing. Afurther, similarly working approach, IEEE 802.1CB, has recently beenstandardized by the IEEE working group TSN. The essential difference isthe fact that the two end nodes (sender and receiver) are not requiredto have particular capabilities because the redundant route in betweenis being configured in the (corresponding) switches. A switch generatesthe two appropriately marked data packets and forwards them from twodifferent ports, while a different switch receives both data packets andforwards an unmarked data packet (corresponds to the original datapacket). As a result, random redundant paths per data packet or perdevice can be defined. Common to all the above-mentioned methods is thetransmitting of at least two independent data packets withoutreconfiguration of the network, which correspondingly consume bandwidthin highly optimized networks. Therefore, these methods have only limitedapplicability particularly in real-time capable data networks.

The following known methods transfer data packets only once, resultingin a more efficient use of the available bandwidth. They differsubstantially with regard to the recognition of a ring break and thesubsequent reconfiguration of the paths.

For example, EP 1 476 988 B1 describes a ring topology with a networknode, which is designed as redundancy manager, connecting the beginningand the end of the ring. The redundancy manager prevents a forwarding ofdata packets, thus acting like an open switch. At regular intervals, theredundancy manager transmits test messages in both directions into thering. If both test messages are once again received by the redundancymanager within a specific interval, an absence of errors in the physicalnetwork is assumed. If not, it can be assumed that the ring topology isinterrupted and from this moment on, the redundancy manager forwardsdata packets (switch closed), whereby the interrupted ring topology istransformed into a functioning line topology. The functionality of theredundancy manager requires a device specifically implemented for suchpurpose, thus standard network devices are not applicable as redundancymanager. Apart from that, the redundancy manager must be arranged at aspecific point of the ring. In addition, the network is here alsoburdened by the test messages, which reduces the bandwidth available forthe actual data communication.

EP 1 062 787 B1 discloses a method, with which, after the occurrence ofan error in the physical network with ring topology, the redundancymanager transmits a data packet to all network nodes, which indicatesthe error. Subsequently, all network nodes delete their source addresstables, causing the network to reconfigure. Through such areconfiguration, the network nodes learn again at which port the mastercan be reached, and the data communication between master and slave canthus be quickly reestablished. Due to the reconfiguration, all networknodes in the ring topology can once again be reached. With this method,all network nodes must have access to their source address table which,once again, requires specifically designed devices and thus no standardnetwork devices can be used in the ring as well.

However, particularly in highly optimized industrial Ethernet-based datanetworks, often so-called cross-traffic is realized, whereby two slavescommunicate directly, i.e. without the inclusion of the master, with oneanother, and as a result, the speed of the data communication can beincreased significantly. In case of an error in the network topology(cable break, disengaging of a plug, defect network node, etc.), thecross-traffic is interrupted at the error location. The aforementionedprior art does not at all go into direct cross-traffic between twoslaves and the quick reconfiguration of the cross-traffic.

Therefore, the problem addressed by the present invention is that ofproviding a method by means of which direct cross-traffic between slavesin a ring topology of the data network can be maintained even in theevent of an error in the ring topology which leads to the interruptionof the ring topology.

According to the invention, this problem is solved such that, in theevent of the occurrence of an error or in case of the rectification ofan error in the annular data network, the ringmaster is configured toforward or block data packets or at least multicast data packets, andthe master prompts the at least two slaves, communicating with oneanother via cross-traffic, to transmit configuration data packets asmulticast data packets to each of the other slaves of the annular datanetwork in order to adjust address tables in the at least two slavescommunicating with one another via cross-traffic.

The method according to the invention does not impose any requirementson the hardware used but builds solely on the standard functionality ofa network switch. As a result, standard network components, particularlystandard unmanaged network switches can be used. The address tables arethus rewritten automatically. The ringmaster is merely a function thatmust be activated in the control software. For implementation, thecommunication protocol must be defined accordingly in order to ensurethat the required messages can be sent and recognized. With the methodaccording to the invention, extremely short switching times in the rangebetween a few 100 μs and few milliseconds can be realized in case of aring break and as a result, the method is also usable particularly inreal-time networks.

When the master transmits multicast data packets in both branches of theannular data network to all slaves present in the ring, areconfiguration of all slaves in the data network can be achieved in asimple manner. As a result, the address tables of the slaves areautomatically adjusted after the beginning or the end of the forwardingof the data packets by the ringmaster (RM), and the traffic between theslaves and the master can be maintained without further losses ornecessary configuration.

An error can be easily recognized if ring status data packets are sentas multicast data packets by the master in temporal intervals in bothbranches of the annular data network, which are received by theringmaster, and the ringmaster detects an error in the annular datanetwork if the ring status data packets are received once or severaltimes in a row from only one branch, or the ringmaster detects therectification of an error in the annular data network if the ring statusdata packets are received once or several times in a row from bothbranches. Such a ring status data packet can be easily implemented inthe communication protocol. These ring status data packets cansimultaneously also be used as multicast data packets for thereconfiguration of the data communication between master and slaves.

Thereby, it is all the more advantageous if normally provided multicastdata packets are used as ring status data packets in the communicationprotocol of the data communication. When building on normally providedand sent data packets, no additional data packets are required whichreduce the bandwidth for the normal data communication.

In a simple manner, the ringmaster can inform the master about an errorif the ringmaster transmits an error data packet to the master.

In the following, the present invention shall be explained in moredetail with reference to FIGS. 1 to 6 which show advantageousembodiments of the invention in an exemplary, schematic, andnon-restrictive manner.

FIG. 1 shows an annular data network with master and ringmaster;

FIGS. 2a and 2b show possible embodiments of a network node with networkswitch;

FIG. 3 shows the annular data network with error;

FIG. 4 shows the notification of the master in the event of an error inthe annular data network;

FIG. 5 shows the reconfiguration of the cross-traffic through thetransmitting of cross-traffic data packets; and

FIG. 6 shows a network topology with network switch with a master and aplurality of annular data networks connected to said network switch.

In a real-time capable Ethernet network protocol, transmission cycleswith predetermined cycle times are defined, in which the master canusually communicate once with every slave S1, . . . , Sn. A transmissioncycle it temporally precisely divided, i.e. the points in time at whichthe master M or the slaves S1, . . . , Sn are allowed to transmit datapackets DP are predefined. As a result, data collisions (or delays dueto accumulating switch queues) at the data network 1 can be prevented.Thus, each of the network nodes involved (master M, slaves S1, . . . ,Sn) knows at which time within a transmission cycle it is allowed totransmit data packets DP. Since Ethernet allows for a full-duplex datacommunication, it is possible that in a network section, data packets DPare transmitted simultaneously in both directions. The real-time-baseddata communication occurs cyclically and a temporal range is providedfor this cyclical (isochronal) data traffic. Therefore, the number ofnetwork nodes, master M and slaves S1, . . . , Sn, and the size of thetransmitted data is also a determining factor for the achievable cycletime. However, in every transmission cycle, a range for asynchronousdata traffic is reserved. The asynchronous data traffic predominantlyserves the data traffic that is not subject to a real-time requirement,and the network nodes present in the data network 1 must share theasynchronous bandwidth according to an implemented pattern.

FIG. 1 shows a simplified annular data network 1. The master M isannularly connected to the slaves via the slaves S1, . . . , Sn. Forthat purpose, the master M is connected to the two branches Z1, Z2 ofthe annular data network 1 and the ends of the two branches Z1, Z2 areconnected to one another via a slave S3 which is provided as ringmasterRM. In FIG. 1, the connection is each indicated by a connection betweenthe ports P11, P12 or P21, P22, etc., of the slaves S1, . . . , Sn.Slave S3 serves as ringmaster RM which, in a flawless annular datanetwork 1, is configured such that no data packets are transmitted froma branch Z1, Z2 to the corresponding other branch Z2, Z1. In FIG. 1,this is indicated by the unconnected ports P31, P32.

A network node K (master M or slave S) is a network device 3 withnetwork switch SW, which can be integrated internally in the networkdevice 3, for example as 3-port network switch as shown in FIG. 2a . Itis also conceivable that a network device 3 is connected to an externalnetwork switch SW in order to form a network node K as shown in FIG. 2b. For the present invention, both realizations of a network node K areconceivable; therefore, in the following, this hardware difference willno longer be elaborated on, and instead it will be assumed that themaster M and the slaves S1, . . . , Sn each are provided with a networkswitch SW. For an annular data network 1, the network switch SW musthave at least two externally accessible ports P1, P2. Often a third portP3 of the network switch SW is connected internally with a control unit2 of the network node K. In the network switches SW, or generally in thenetwork node K, an address table AT is also implemented in a knownmanner, from which the network switch SW extracts the information as toby which of the external ports P1, P2 a different network node of theconnected data network can be reached.

A network node K can also have additional external ports P33, asindicated in FIG. 1 on slave S3, to which further network nodes Kn canbe connected. However, these further network nodes Kn are not consideredto be part of the annular data network 1. Such a further external portP33, however, can be reachable from both branches Z1, Z2, indicated bythe dotted connection in FIG. 1, which does not close the ring.

In the course of the data communication on the annular data network 1,often so-called multicast data packets are also transmitted, whereby anetwork node (master M, slaves S1, . . . , Sn) transmits a data packetto a plurality of other network nodes in the annular data network 1.Such a multicast data packet is received at a port of the network nodeand retransmitted on the other port or other ports. In order to preventthe uncontrolled circulation of such multicast data packets in theannular data network 1, the ringmaster RM, in case of an intact annulardata network 1, is configured such that it at least does not forwardmulticast data packets. Preferably, the ringmaster RM, in case of anintact annular data network 1, is configured such that it does forwardno data packets at all, including unicast data packets.

Within the framework of the data communication at an intact annular datanetwork 1, the master M transmits, for example, data packets DP1, DP2into both branches Z1, Z2 of the data network 1 in ring topology. Thedata packets DP1, DP2 are either transmitted to exactly one of theslaves S1, . . . , Sn (unicast traffic) or to a plurality of or even allslaves S1, . . . , Sn, or to a plurality of or even all slaves S1, . . ., Sn of a branch Z1, Z2 (multicast traffic). In multicast traffic, thedata packets DP1 and DP2 can also be identical. Corresponding to theimplemented communication protocol, the slaves S1, . . . , Sn transmitdata packets back to the master M in accordance with the communicationprotocol.

As ringmaster RM, a slave is selected ideally in the center of the ring,i.e. a slave that in both branches Z1, Z2 is approximately at equaldistance from the master M in order to achieve approximately the sametransmission times in both branches Z1, Z2.

In the event of an error F, as shown in FIG. 3, the annular data network1 is interrupted at the error location. An error F can be detected bythe ringmaster RM if the master M transmits at regular intervals aspecific ring status data packet DPR as multicast data packet to allslaves S1, . . . , Sn. The ringmaster RM can thus recognize whether thearrival of such a ring status data packet DPR at one of its ports P31,P32 does not occur and, in such case, can extrapolate that there is anerror F in one of the corresponding branches Z1, Z2. Of course, in theringmaster RM, it is possible to configure how often the successivenon-occurrence of the ring status data packet DPR must be detectedbefore an error in the annular data network 1 is assumed to haveoccurred. In error-prone data networks 1, for example with error-pronetransmission lines such as slip rings, WLAN lines, etc., a higher valueis preferably set.

In order to avoid additional burden on the data communication throughthe transmission of the ring status data packets DPR, it is possible usedata packets already provided in the communication protocol for suchpurpose. For example, the communication protocol can contain multicastdata packets from the master M to the slaves S1, . . . , Sn that aretransmitted in specific or all transmission cycles, preferably in thecyclical part of the transmission cycle. The ringmaster RM can thusexpect the arrival of these multicast data packets and assume an error Fin branch Z1, Z2, from which no data packet is received. The use of suchmulticast data packets from the master M as ring status data packets DPRis further advantageous because, in case of an error detection, theconnection in the ringmaster RM that was open thus far can beimmediately closed; as a result, the multicast data packets of themaster can be forwarded via the ringmaster RM. The address tables of theslaves connected behind the ringmaster RM (to the master) withoutrequiring further measures.

Such multicast data packets from the master M to the slaves S1, . . . ,Sn usually also contain data from the master M to the slaves S1, . . . ,Sn. This simultaneously ensures that the data reach all slaves S1, . . ., Sn even in the event of a ring break and that all slaves S1, . . . ,Sn receive their data in the same transmission cycle, in which theswitch occurs. Data loss and delay due to retransmitting of the data canthus be prevented.

Of course, the method for detecting an error F can also be reversed. Inthis case, the ringmaster RM can be configured to transmit at regularintervals a ring status data packet DPR via both branches Z1, Z2 to themaster M. In the event that no ring status data packet DPR is received xtimes, the master M can once again extrapolate that there is an error Fin the corresponding branch Z1, Z2.

However, the master M can also detect an error without a separate ringstatus data packet DPR. For such purpose, the master M can again use thedata packets already provided in the communication protocol. Forexample, if one or each slave S1, . . . , Sn transmits a data packet tothe master M in every transmission cycle, the master M can deduce anerror F if said data packets are not received. If the master M usuallyreceives a data packet from each slave S1, . . . , Sn, the master candeduce an exact error location even on the basis of the missing datapackets, provided that the master M knows the topology of the datanetwork 1 (which is usually the case).

Here it must be noted that on the physical layer of a network deviceoften there is a link detection is implemented as well, as is the case,for example, with Ethernet. However, this link detection is not reliableenough because it is possible that a lost link (e.g. due to a corebreakage in the network cable) is not detected. Apart from that, thislink detection would also not be sufficiently quick because, forexample, with Ethernet a pulse is transmitted every 16 ms and the pulsewould have to fail several times in a row at the receiver side for thelink detection to respond.

Once the ringmaster RM or the master M has detected an error F in thedata network 1 in this or any other suitable way, the connection betweenthe ports P31, 32 of the ringmaster RM is closed (FIG. 3), thus makingdata communication (both as unicast and multicast) via the ringmaster RMpossible. Simultaneously, a method for reconfiguring the data network 1is initiated, e.g. similar to the initially described prior art.

When the master M transmits multicast data packets to the slaves S1, . .. , Sn for reconfiguration, reconfiguration is effected automatically bythe standard Ethernet functionality without the requirement of aspecific method for reconfiguration or imposing specific hardwarerequirements on the slaves S1, . . . , Sn. Such multicast data packetscan be transmitted in intervals or only once. For example, the ringstatus data packets DPR can also be used for such purpose.

Due to the reconfiguration (regardless of the method used), each slaveS1, . . . , Sn can again be reached by the master M and each slave S1, .. . , Sn can reach the master M. Within the course of thisreconfiguration, the address tables AT1, . . . , ATn of the slaves S1, .. . , Sn and the address table ATM of the master M are reconfiguredcorrespondingly, which shall be explained with the example of theaddress table ATM of the master M.

FIG. 1 shows the address table ATM of the master M, which shows whichport PM1, PM2 of the master M is required to reach each slave S1, . . ., Sn of the annular data network 1. In the course of the reconfigurationin the event of an error F, the address table ATM of the master M isrewritten, and so the address table ATM now contains that the slave S2can no longer be reached via the port PM1 but instead via the port PM2(FIG. 3). The address tables AT1, . . . , ATn of the slaves S1, . . . ,Sn are rewritten in the same manner by the reconfiguration. For example,slave S2 no longer transmits data packets DP to the master M via theport P21 but instead via the port P22. In this manner, the normal datatraffic between master M and slaves S1, . . . , Sn can be quicklyreorganized. For this purpose, any method for the reconfiguration of thedata communication between master M and the slaves S1, . . . , Sn isapplicable.

However, in the annular data network 1, a direct cross-traffic betweenslaves S1, . . . , Sn can also be implemented. Cross-traffic in thiscontext means that two slaves communicate with one another via the datanetwork 1 without the inclusion of the master M by exchangingcross-traffic data packets DPQ among one another. For example, it can beprovided that the slaves S1, S2 exchange cross-traffic data packets DPQwithout the inclusion of the master M, as indicated in FIG. 3. Theslaves S1, . . . , Sn which communicate directly with one another do nothave to be adjacent to one another; instead, the cross-traffic couldalso be implemented across a plurality of slaves S1, . . . , Sn. Thecross-traffic is also configured by appropriate entries in the addresstables AT of the slaves S1, S2 involved in the cross-traffic, asindicated in the address tables AT2 of slave S2.

This cross-traffic allows the slaves involved to directly communicatewith one another within a transmission cycle and independently from theremaining data communication (except for avoiding collisions or possibledelays due to switch queues at the data network) by means of exchangingdata packets. This allows for very quick reactions of a slave S1, S2involved in the cross-traffic, which is particularly interesting inhighly dynamic, synchronized controls of machines. For example, directcross-traffic between a sensor (for example, a rotation sensor) and amotor control of an electric motor can be provided in a drive controlwhich would allow for the drive to be operated with a very shortscanning time (corresponds to the time of a transmission cycle). Ifcommunication via the master were necessary, then the possible scanningtime would be correspondingly longer due to the longer communicationpaths.

However, with the reconfiguration of the data network 1 in the course ofan error F as described above, said cross-traffic would be interruptedat the error location.

If, due to the reconfiguration, only those parts of the address tablesAT1, . . . , ATn of the slaves S1, . . . , Sn are rewritten which relateto the data traffic with the master M, entries in the address tablesAT1, . . . , ATn remain which would prevent a cross-traffic. Forexample, if the entry that slave S1 can be reached by slave S2 via portP21 remains in the address table (as in FIG. 3), slave S2 wouldunsuccessfully attempt to transmit cross-traffic data packets DPQ viathis port P21 to slave S1 because the error F is present between theslaves S1, S2. The same can occur if, in the course of thereconfiguration, the entire address tables AT1, . . . , ATn are deleted,thus also deleting the entries for the direct cross-traffic. However,these entries required for the cross-traffic are not rewritten by thereconfiguration according to the prior art, which would also prevent thedirect cross-traffic via the error location.

In order to circumvent this problem, it is provided, according to theinvention, that, in the event of a detection of an error F in theannular data network 1 via the intact branch Z2 of the annular datanetwork 1, the ringmaster RM transmits an error data packet DPF to themaster M (FIG. 4) in order to inform the master M about the error F. Ifthe error F is detected in the master M, the master M can transmit theerror data packet DPF to the ringmaster RM, making it possible for theringmaster RM to close the data communication connection via its portsP31, P32, as described above. The bandwidth for the error data packetDPF can be fixedly implemented in the transmission cycle, preferably inthe isochronal part of the transmission cycle in order to be able torelease the notification as soon as possible without the wait for a freetransmission slot.

However, in principle, the method according to the invention forreconfiguring the direct cross-traffic between two slaves S1, . . . , Snis independent from the manner in which an error F is detected in theannular data network 1.

Once an error F is detected in the annular data network 1 and the masterM is informed about such detection, the master M transmits a start datapacket DPN to all slaves S1, . . . , Sn (FIG. 5), with which all slavesS1, . . . , Sn of the annular data network 1 are prompted to transmit aconfiguration data packet DPMC in the form of a multicast data packet toeach of the other slaves S1, . . . , Sn (FIG. 5). This means that eachslave S1, . . . , Sn transmits the configuration data packet DPMC viaall ports P that are integrated in the annular data network 1. Forexample, slave S2 transmits its configuration data packet DPMC2 via theports P21, P22. On a slave S1, . . . , Sn, additional ports P notintegrated in the annular data network 1 can be provided, to which theother network nodes Km are connected. The slave Sn in FIG. 5, forexample, has an additional external port Pn3, to which a further networknode Km is connected. For the method according to the invention, it isnot required that a configuration data packet DPMCn is also transmittedvia such ports P which are not integrated in the annular data network 1.

If the master M is aware of the configuration of the cross-traffic,i.e., if the master M knows which of the slaves S1, . . . , Sn areconfigured for cross-traffic among one another, it also suffices if themaster transmits the start data packet DPN only to the slaves S1, . . ., Sn which participate in the cross-traffic. Thus, only the slaves S1, .. . , Sn which participate in the cross-traffic would transmit such aconfiguration data packet DPMC.

In order to make configuration data packets DPMC in the form ofmulticast data packets possible, the ringmaster RM, in the event of anerror, must forward multicast data packets from slaves S1, . . . , Sn.Therefore, in case of an error, the ringmaster RM must remove therestriction required for the intact ring. However, the error F (ringbreak) ensures that these multicast data packets DPMC cannot circulateuncontrolledly. The slave S1 thus receives the configuration data packetDPMC2 of the slave S2 via port P11 (FIG. 5). By means of the standardmechanisms of an unmanaged network switch, the address table AT1 in theslave S1 is automatically rewritten if the slave S1 receives a datapacket of another slave S2 via another port P11 which is different fromthe one recorded in the address table AT1. Due to said automaticrewriting of the address table AT1, slave S1 knows that slave S2 can nolonger be reached via port P12 but instead via port P11. Due to theconfiguration data packet DPMC1 of slave S1, the address table of slaveS2 is rewritten in the same manner, and so slave S2 also knows the slaveS1 can be reached via port P22. Thus, the cross-traffic via the errorlocation F can be circumvented and the cross-traffic between the slavesS1, S2 takes place via the intact connections of the annular datanetwork 1. As a rule, no interference with the functionality of thenetwork switch of the slaves S1, . . . , Sn is hereto required becausesaid rewriting of the address tables AT is a standard function of anetwork switch. The rewriting of the address tables AT and thus also thereconfiguration of the cross-traffic is effected automatically.

If the error F is rectified, the procedure is similar. For example, theringmaster RM or the master M recognizes for example that ring statusdata packets DPR once again arrive from both branches Z1, Z2, which isonly possible if the error location has been rectified. The ringmasterRM informs the master M (or vice versa) that the error F has beenrectified, for example, once again by an error data packet DPF. Uponrecognizing the rectification of the error F, the ringmaster RM againprevents at least the forwarding of multicast data packets of the slavesS1, . . . , Sn in order to prevent an uncontrolled circulation of suchdata packets in the ring. Preferably, the ringmaster RM prevents theforwarding of all data packets. Via a start data packet DPN, the masterM again prompts all slaves S1, . . . , Sn, or advantageously all slavesS1, S2 participating in the cross-traffic, to transmit a configurationdata packet DPMC to every other slave S1, . . . , Sn. Theseconfiguration data packets DPMC as multicasts are not forwarded by theringmaster RM. As a result, in the example according to FIG. 5, theconfiguration data packet DPMC2, at corrected error F, would arrive atslave S1 via port P12 which would result in the rewriting of addresstable AT1. The same would apply to address table AT2 of slave S2 whichwould be rewritten due to a configuration data packet DPMC1 of slave S1.The cross-traffic between slave S1 and slave S2 would again be effectedalong the shorter path via the ports P12 and P21.

The start data packet DPN is preferably transmitted as multicast datapacket; i.e. a slave S1, . . . , Sn receives the start data packet DPNat a port and forwards it via this or the other port(s). Alternatively,the master M could also transmit individual start data packets DPN toslaves S1, . . . , Sn, which would utilize more bandwidth of the datacommunication.

With an intact ring R, i.e. without error F, it is also possible toeliminate cross-traffic via the ringmaster RM. The reason lies in thefact that such cross-traffic, e.g. between slaves S2 and S4, could beconfigured but not maintained. The configuration can be effected bymeans of a configuration tool when setting up the data network 1.However, the ringmaster RM could also fake cross-traffic in the form ofdata packets in order to rewrite the address tables AT2, AT4 of theslaves S2, S4 correspondingly. But the configuration of cross-trafficvia the ringmaster RM can only be maintained until one of the slaves S2,S4 involved transmits multicast data packets, which may occurfrequently. Since the ringmaster RM blocks such multicast data packetsin the intact ring R, the multicast data packet would reach the otherslave via the long path, resulting again in a rewriting of the addresstables AT2, AT4. Therefore, it is meaningful to completely eliminatecross-traffic via the ringmaster RM.

The normal data communication (i.e., not the direct cross-trafficbetween S1, . . . , Sn) after correction the error could again bereconfigured with conventional methods. However, for thereconfiguration, the master M again could transmit multicast datapackets to the slaves S1, . . . , Sn, resulting in the automaticreconfiguration by the standard Ethernet functionality without therequirement of a specific method for reconfiguration or imposingspecific hardware requirements on the slaves S1, . . . , Sn. Suchmulticast configuration data packets can be transmitted in intervals oronly once. For example, the ring status data packets DPR can also beused for such purpose.

Of course, the master M could also be integrated in the ring topology bymeans of an external network switch SW, as described in FIG. 6. In thiscase, a network switch SW is provided which is connected to the masterM. A first ring R1 in the form of an annular data network 1 with anumber of slaves S11, . . . , S1 n and a second ring R2 in the form ofan annular data network 1 with a number of slaves S21, . . . , S2 m isconnected to the network switch SW. In addition, other network segmentsNS can also be connected to the network switch SW, for example, a linearnetwork segment with two slaves S3, S4 as in FIG. 5. In this manner,even a plurality of annular data networks 1 can be operated with amaster M. However, the basic method of handling an error F remainsunchanged, and each of the rings R1, . . . requires a slave withringmaster function.

It is also conceivable that the function of the ringmaster RM is assumedby the master M. In such case, the error data packet DPF would not betransmitted via the annular data network 1 but internally signaled bythe master M. This does not change the basic procedure of reconfiguringthe cross-traffic.

Furthermore, it must be emphasized again that the method forreconfiguring the cross-traffic according to the invention isindependent from the method for reconfiguring the data traffic betweenmaster M and the slaves S1, . . . , Sn.

The configuration data packets DPMC of the slaves S1, . . . , Sn forreconfiguring the cross-traffic are preferably, but not necessarilyrealized as a synchronous data traffic. It is thus possible to completethe reconfiguration within a few transmission cycles, ideally within onetransmission cycle, which allows for a particularly quick switchover ofthe cross-traffic. Hereby, a slave S1, . . . , Sn would receive thestart data packet DPN in one transmission cycle and transmit theconfiguration data packets DPMC in the asynchronous range in the sametransmission cycle. In this case, it is particularly ensured that thecross-traffic is reorganized before the next transmission cycle begins,whereby data loss due to data packets lost at the error location F canbe prevented, which is particularly important in a real-time capabledata network.

If the ring status data packet DPR is transmitted as multicast datapacket in the isochronal part of the transmission cycle (and thustemporally precisely planned) and a threshold value is set as to howmany data packets must be lost before a switch is effected, then it ispossible, in connection with the knowledge as to how long thetransmitting of the error data packet DPF and the configuration datapackets DPMC lasts, to make a precise estimation as to how long it takesuntil the ring, in case of an error, is active again without data packetloss.

However, the predetermined cycle time also influences the maximum sizeof the ring R. In case of an error, the cross-traffic via the errorlocation cannot be maintained but must be routed around such errorlocation, resulting in longer travel times of the data packets of thecross-traffic. Therefore, the number of network nodes in the ringmultiplied by the corresponding latency period of the network nodes(i.e., the time that a network node requires to transmit a data packet,which arrives at a first port, to a second port) must be less than thecycle time. If this requirement is not met, the cross-traffic cannot bemaintained in case of an error because it might not be possible toconclude the cross-traffic within the cycle time.

A further limitation with regard to the permissible size of the ring Rmay arise if the reconfiguration is to be completed within atransmission cycle. Within a transmission cycle, a master M can transmita start data packet DPN only to a specific number of slaves S1, . . . ,Sn. Furthermore, only a specific number of slaves S1, . . . , Sn cantransmit their configuration data packets DPMC within the transmissioncycle in the asynchronous range of the transmission cycle. Both limitthe possible number of network nodes in the ring R if the cross-trafficis to be switched particularly fast, i.e. within a transmission cycle.

1. Method for data communication in the form of cross-traffic between atleast two slaves (S1, S2) of an annular data network (1), wherein amaster (M) is connected to a first branch (Z1) of the annular datanetwork (1) with a number of slaves (S1, S2) and to a second branch (Z2)of the annular data network (1) with a number of slaves (S4, Sn), andthe ends of the branches (Z1, Z2) are connected to a slave (S3) providedas a ringmaster (RM), characterized in that, in the event of an error(F) in the annular data network (1), the ringmaster (RM) is configuredto forward data packets, and that the master (M) prompts the at leasttwo slaves (S1, S2), communicating with one another via cross-traffic,to transmit configuration data packets (DPMC1, DPMC2) as multicast datapackets to each of the other slaves (S1, . . . , Sn) of the annular datanetwork in (1) order to adjust address tables (AT1, AT2) in the at leasttwo slaves (S1, S2) communicating with one another via cross-traffic. 2.Method for data communication in the form of cross-traffic between atleast two slaves (S1, S2) of an annular data network (1), wherein amaster (M) is connected to a first branch (Z1) of the annular datanetwork (1) with a number of slaves (S1, S2) and to a second branch (Z2)of the annular data network (1) with a number of slaves (S4, Sn), andthe ends of the branches (Z1, Z2) are connected to a slave (S3) providedas a ringmaster (RM), characterized in that, in the event of therectification of an error (F) in the annular data network (1), theringmaster (RM) is configured to block at least multicast data packets,and the master (M) prompts the at least two slaves (S1, S2),communicating with one another via cross-traffic, to transmitconfiguration data packets (DPMC1, DPMC2) as multicast data packets toeach of the other slaves (S1, . . . , Sn) of the annular data network(1) in order to adjust address tables (AT1, AT2) in the at least twoslaves (S1, S2) communicating with one another via cross-traffic. 3.Method according to claim 1, characterized in that the master transmitsmulticast data packets in both branches (Z1, Z2) of the annular datanetwork (1) to all slaves (S1, . . . , Sn) present in the ring. 4.Method according to claim 1, characterized in that ring status datapackets (DPR) are transmitted as multicast data packets by the master(M) in intervals in both branches (Z1, Z2) of the annular data network(1), said ring status data packets (DPR) being received by theringmaster (RM), and that the ringmaster (RM) detects an error (F) inthe annular data network (1) if the ring status data packets (DPR) arereceived once or several times in a row from only one branch (Z1, Z2),or that the ringmaster (RM) detects the rectification of an error (F) inthe annular data network (1) if the ring status data packets (DPR) arereceived once or several times in a row from both branches (Z1, Z2). 5.Method according to claim 4, characterized in that multicast datapackets, provided in the communication protocol of the datacommunication, are used as ring status data packets (DPR).
 6. Methodaccording to claim 4, characterized in that the ringmaster (RM)transmits an error data packet (DPF) to the master (M) in order toinform the master (M) about the occurrence of an error (F).
 7. Methodaccording to claim 1, characterized in that the master (M) transmits astart data packet (DPN) in order to prompt the at least two slaves (S1,S2), communicating with one another via cross-traffic, to transmit theconfiguration data packets (DPMC1, DPMC2).